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Re: Identifying Pure and Const Functions

I had a look at the file ipa-pure-const.c. I have wriiten a Simple IPA
Pass which identifies pure and const functions in the program. I have
inserted this pass after ipa-pta pass which is executed after
ipa-pure-const pass. Also, the option -fipa-pure-const is enabled.
I am using DECL_PURE_P(t) and TREE_READONLY(t) to identify whether t
is a pure or a const function.

I tested this code on a sample program which consists of a function
"error" which returns a constant value. gcc fails to identify it as a
pure function. However, if I specify the function as pure as shown
below, gcc identifies it as pure function

int error()
        return 2;

I am unable to understand why error function is not identified as a
pure function by gcc?  Are the checks to identify pure and const
functions correct?

Pritam Gharat

On Fri, May 29, 2015 at 1:26 PM, Marek Polacek <> wrote:
> On Fri, May 29, 2015 at 01:16:32PM +0530, Pritam Gharat wrote:
>> How do we identify whether a function is a pure or a const function?
>> Is there any flag associated with its cgraph_node or the tree node
>> (decl of cgraph_node)?
> You'll want to look into ipa-pure-const.c.
>         Marek

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